The invention relates to a method and a device for monitoring and/or controlling and regulating a granulation, agglomeration, instantization, coating and/or drying process in a fluidized bed or a moving bulk by determining the product moisture.
In the prior art indirect methods are known for determining the product moisture, for example using a balancing of the inlet and outlet streams from which the product moisture is determined. The inlet and exhaust air conditions (temperature and humidity) are measured, as well as the volume flow of the inlet and exhaust air and the spray rate. From many different measuring positions (a minimum of seven measurement quantities), a correspondingly large number of measurement errors results and thereby a product moisture that is not exactly specified. Problems occur especially for processes which require a fast regulation, for example the detection of the shut-off point for spraying during granulation. An indirect measurement of the humidity of the product environment can be performed by a capacitive measuring sensor which is positioned directly in the fluidized bed. Generally, the actual measurement sensor is surrounded by a water vapor-permeable protection cap, through which the water vapor must diffuse, which is then correspondingly detected by the sensor.
It is problematic therein that this involves a slow measuring method. During granulation, an air atmosphere always appears in the product environment which is almost completely saturated with water, which can lead to condensation on the sensor (so-called over-moistening of the sensor). The condensed water requires a considerably longer time until it is released again to the outside via the protection cap. During this time, the measurement signal always lies at about 100% air humidity. Furthermore, contamination of the sensor can lead to considerable measurement error. In addition, as a result of the partly strong abrasive forces by the fluidized product, the sensor only has a short service life.
Using NIR-spectroscopy, only the surface moisture can be measured. Usually, a very high calibration expense and an expensive mathematical evaluation and averaging of the measurement signal are necessary for this. The IR-sensor can be integrated in the fluidized layer only via a window that is flush with the wall, so that problems result from glazing of the window, because this leads to the immediate reflection of radiation. A measurement is then ruled out. Furthermore, due to the short wavelengths and the high reflection of the radiation thus present, there is no penetration depth into the fluidized layer or the product. Thus, only the surface moisture can be recorded. Finally, the dependence of the measurement signal on the grain size and on the bulk density, and the dependence of the measurement signal on the color change and the thereby resulting high calibration expense, are disadvantageous.
A measurement of the moisture can also be performed via an impedance measurement with two electrodes on an electrically insulating substrate (PTFE), which are connected together to be electrically conducting via a moisture-sensitive layer of an electrolytic solid (water vapor permeable). An equilibrium humidity is measured, which appears in the product environment. The direct product moisture is not measured. The service life of the sensor in the fluidized layer is very unsatisfactory.
Furthermore, it is known to record the levels of inlet and exhaust air temperature, volume flow, inlet and exhaust air humidity, and the spray rate. From these quantities, however, generally only indirect information can be derived about the actual product or the water content appearing in the product. Exact readings are generally only possible with off-line measurement methods, such as dry weight determination, vacuum drying chamber, desiccator method, infrared scales, or by the Karl-Fischer titration. These methods are, however, very time consuming and require a time expenditure of 10 minutes to half a day for a moisture measurement, so that these methods permit no real-time or current information for the process.
A further possibility for detecting the result or the progression of a granulation process is represented by the possibility of directly determining or measuring the particle size or the grain size distribution during the process. The processes known for this, such as ultrasound measurement or laser diffraction spectroscopy, are characterized by a very high cost of devices and by extremely complicated mathematical and statistical evaluation procedures. For the measurement, defined measurement sections, for example a defined bypass for the product, are usually necessary, such that an insertion directly into the fluidized layer is not possible. A reproducibility of the method is generally achieved only under certain conditions.
With all of the above-mentioned known processes for controlling or regulating the processes, it is disadvantageous that they are either too slow and/or too susceptible to disturbances and/or not accurate enough, and therefore a limited practical suitability is available only for certain products.
An object of the present invention is to create a method of the type mentioned at the beginning, as well as a device, whereby under the more difficult conditions that are prevalent with these processes, in particular with respect to the accessibility of the product, the contamination and the service life of the sensor, etc., the product moisture can be measured reliably with higher measurement accuracy, and this measurement quantity can be used for the control or the regulation of the process. In particular, this moisture measurement should provide a result that can be used directly as a measure for certain product characteristics. Moreover, the measurement should also be applicable for different products without costly adaptation and calibration.
In order to achieve this object, it is proposed that, at least during one segment of the process, the total product moisture is measured substantially continuously at least over a period of seconds, in a contact-free manner using electromagnetic radiation in the high frequency or microwave range, by evaluation of the attenuation as a measure of this total product moisture, and taking into consideration the product temperature, the total product moisture is held in a pre-determined range via a control circuit by changing the spray rate and/or the gas temperature and/or the volume flow.
The invention is based upon the discovery that the total product moisture, and consequently both the surface moisture as well as the moisture present within the product in the capillaries or the cavities of a granulate grain, is a key piece of information, by which an exact influence of the respective process is possible.
From European published patent application EP-A-0 403 820 a process for drying a bulk is indeed known, in which an electromagnetic radiation is used in order to dry the bulk and to measure its moisture. The electromagnetic radiation is used therein for drying the bulk, wherein the microwave device functions as an active drying device, while the reflected output is only measured in order to determine the end of the drying process. However, no indication is contained therein that the electromagnetic radiation is used only for measurement and that this measurement is used to control the total product moisture by adjustment of the spray rate.
The method according to the invention and the corresponding measurement device constitute an improvement of a conventional process, in which the drying is not influenced by the microwave radiation, so that measurement data are available about the actual, directly present total product moisture, which can thus be used directly, i.e., xe2x80x9conlinexe2x80x9d, as process-accompanying measurement quantities for the control of the directly on-going process. The device according to the invention makes it possible to measure the product moisture within very narrow limits, exactly and reproducibly. By the continuous on-line detection of the product moisture, one obtains, for example during a granulation process, a reproducible grain size distribution wherein the total product moisture is practically a direct measure for the grain size distribution.
The following embodiments make clear the interdependence between the total product moisture and certain product characteristics.
Via the interfacial forces and the capillary pressure on freely moving liquid surfaces, the necessary cohesion occurs between the product grains for the granulate formation from several grains . A distinction must be made therein whether the cavities between the individual grains are filled with liquid only partially (formation of liquid bridges between the grains) or completely (formation of capillary adhesive forces). Likewise, several solid grains can be surrounded by a complete liquid skin, whereby surface tension forces can lead to an agglomeration of several drops. As a result of these mechanisms, with moist granulation there occurs both a defined grain enlargement and a defined reduction of already existing granulates. Besides product-specific properties, such as wettability, porosity and hygroscopicity, a significant influence on the granulation results from the absolute content of product moisture. The product moisture results from the adjusted spray rate and the drying output in the course of the granulation (between the particle sizes, the spray rate and the particle grain).
For the size growth rate, and thus also for the grain size distribution of the granulation, the following functional relationship exists for a moist granulation essentially via the particle surface available for the exchange:
The granulation occurs through adherence of particles, not yet been granulated or less strongly granulated, on the still moist particle surface of other, mostly larger particles. The larger the particle surface relative to volume (i.e., the smaller the particles), the larger the drying capacity. The larger the particles, the smaller the particle surface relative to volume and thus the smaller the drying capacity. Toward the end of the granulation an over-moistening of the product can easily occur in the case of a constant spray rate, which can lead to uncontrolled agglomeration and process interruption. The total product moisture thus represents a significant parameter for controlling and regulating a granulation process. Because of the relationship shown, a reproducible granulation mechanism results. The product moisture is accordingly a direct measurement. The product-related result can be characterized by the grain size distribution.
There is also the possibility that, via the measurement of the total product moisture in a granulation/coating process, the maximum permissible product moisture is determined for maintaining a stable and homogeneous fluidization, and/or a pre-determinable, constant product moisture is regulated and/or the end point of the granulation is determined.
Furthermore, the possibility exists that, via the measurement of the total product moisture during a drying process, the end of the process is determined during drying at a desired end moisture content. This possibility is particularly usable when the drying represents the last phase of a granulation process for reaching a pre-determined end moisture.
Finally, by the measurement of the total product moisture, a pre-determinable moisture progression can be regulated via a direct correlation between the spray rate and the product moisture.
Special advantages also result in use in the context of the so-called xe2x80x9cscale-up.xe2x80x9d In this regard, for a calibration in the context of a process adjusted to the laboratory standard, and the adaptation of the process based thereupon to an actual production standard, the adjustment of the spray rate exclusively via the measurement of the total product moisture is performed as a quantity that is not a function of the instruments. In particular, during this scale-up, an instrument-independent quantity is available with the total product moisture measured using electromagnetic radiation, which quantity as a measure makes possible the transfer of process conditions from small scale to large scale.
The invention also relates to an aeration device, in particular a fluidized layer apparatus with a container and an impermeable seal, which has a sealing part insertable in a container wall opening, wherein for the process control a control circuit is provided at least with one temperature sensor as well as mechanisms for changing the spray rate and/or the gas temperature and/or the volume flow.
This device is characterized in that in the impermeable seal, a microwave sensor is integrated as a moisture sensor, which is connected via an electric cable to an evaluation device as part of the process control, that the impermeable seal has a sensor holder formed by a carrier member, and that the carrier member or the moisture sensor is arranged for a seal to the container wall that is approximately flush with the inner wall and is free of dead spaces.
With an aeration device of this type the method can be performed especially well, because when using a microwave sensor, among other things, special sampling devices having direct access into the fluidized bed or coverings are not necessary.
The use of the microwave sensor in connection with the impermeable seal in a dead space-free arrangement has the essential advantage that the system and the components usedxe2x80x94impermeable seals, sensorsxe2x80x94can be easily cleaned, which is also possible when the system is closed. In connection with the provided impermeable seal, the sensor can still be retrofit in a simple manner later.
It is also advantageous that the microwave sensor installed in the impermeable seal is connected only via a cable as the control line to the evaluation device. The length of the cable therefore plays practically no role, so that the evaluation device connected thereto can be arranged set apart from the fluidized layer apparatus. This is a considerable advantage, because in the area surrounding the fluidized layer there is often little space available, and because the devices arranged in this direct production area must be cleaned according to exactly prescribed steps, which in the present case is not applicable.
Expediently, the carrier member has a recess for receiving the microwave sensor and is preferably constructed as a protection cap for the sensor. The cap is preferably made of polytetrafluoroethylene (PTFE) and closes off the container inner wall in an approximately flush manner. The sensor is thereby housed in a well-protected manner, and by the carrier member that seals in a flush manner with the container interior, the container inner wall is continued in an approximately continuous manner, without disruptive parts projecting into the container interior. In order to be able to replace the microwave sensor easily, when necessary, a detachable cover flange is provided for the detachable mounting of the microwave sensor. The sensor, which can be constructed, for example, as a planar sensor, can be mounted flush with the wall in the container wall in the same manner as a port hole.
Expediently, relative to a pre-determined rest bed-dumping height of the respective product within the container, the microwave sensor is arranged at a height of approximately up to two times the rest bed-dumping height, preferably at the upper edge area of the rest bed within its rest bed-dumping height. On the one hand, the microwave sensor is thus arranged at a well-accessed position and, on the other hand, especially good measurement results arise with the arrangement at this height, because in this measurement position the fluidized bed is detected in a representative area.